The present disclosure relates generally to integrated circuits, and more particularly, an oscillator bias stabilization circuit for single-pin crystal oscillators with fast start-up capability.
Modern clocking circuits in integrated circuits require stable frequency references (i.e., oscillators) with fast turn-on, small output noise, and low power consumption. Because of stringent frequency stability requirements, an external resonator with a high-quality factor (i.e., a quartz crystal) is typically employed in conjunction with active components in the integrated circuit for generating the reference oscillations. However, because the number of pins available in an integrated circuit for interfacing with other external circuits is limited, it is advantageous to use only one pin for connecting the resonator. Consequently, circuit implementations of oscillators that satisfy the foregoing requirements are challenging, especially in low-supply-voltage environments where the active devices (i.e., transistors) must operate with small headroom voltages.
An undesirable characteristic in such circuits can arise due to component mismatches that cause variations in the common-mode voltage (bias) of the circuit. These variations can be large enough during start-up so as to cause transistors in the active core of the oscillator to operate in a non-linear region (i.e., the triode region of MOS transistors), thereby reducing the oscillator loop gain and increasing the start-up time.
Accordingly, there exists a need for an oscillator bias stabilization circuit that prevents undesirable bias variations in the circuit during start-up to ensure sufficient loop gain and fast start-up time.